Technical Field
The present invention relates to a filter circuit and to a wireless receiver.
Background Art
Wireless receivers use filter circuits that pass desired signal and attenuate noise. Complex bandpass filters are known as analog filters that are used in wireless receivers (referencing Japanese Unexamined Patent Application Publication 2008-205962, Japanese Unexamined Patent Application Publication 2008-211531, and Japanese Unexamined Patent Application Publication 2012-54684). The complex bandpass filter uses, as input signals, a Q signal and an I signal that are intermediate frequency signals with a 90° phase difference therebetween, to pass the desired signal and attenuate the noise, depending on desired paint-pass characteristics. Moreover, the complex bandpass filter operates so as to attenuate image response (image noise) through passing input signals in which a phase difference between the Q signal and the I signal is 90° and attenuating input signals in which the phase difference is −90°.
Note that in recent years, miniaturization of semiconductor elements has enabled digital processing to be achieved with smaller footprints and reduced electrical power, digital circuits have been used in addition to conventional analog circuits. However, achieving the replacement of all analog filter processing with digital filter processing would be extremely difficult from the perspective of cost, and the like. Given this, a filtering process is carried out in an analog circuit wherein the number of stages of the filter is reduced, and, thereafter, those processes which were inadequate are supplemented through digital circuits.